Asymmetric transfer molding method and an asymmetric encapsulation made therefrom

ABSTRACT

A method of encapsulating an article having first and second surfaces, includes positioning a first molding section in a sealing relationship with the first surface of the article and positioning a second molding section adjacent the second surface of the article. The first molding section is filled first thereby forcing the second surface of the article into a sealing engagement with the second molding section. The second molding section is then filled.

This application is a continuation of prior U.S. application Ser. No.09/388,045 filed Sep. 1, 1999, now U.S. Pat. No. 6,605,331, which is adivisional of prior U.S. application Ser. No. 09/255,554, now U.S. Pat.No. 6,143,581, filed Feb. 22, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of encapsulating an article and, morespecifically, to transfer molding an asymmetric encapsulation for anelectronic device, and an asymmetric encapsulated electronic device.

2. Description of the Background

It is well known that electronic devices are sensitive and thus requireprotection from physical and environmental conditions which may degradeor completely ruin them. Therefore, it is well known in the art toprotect electronic devices from these conditions by sealing them with aprotective encapsulation material. This “packaging” of the electronicdevices protects them from the conditions which may degrade them andallows the devices to be transported and handled, and thus allows themto be easily configured with other components. Several encapsulationmethods are known in the art such as under-fill encapsulation (for flipchip applications) and glob topping.

One prior-art method for encapsulating devices is the “transfer-molding”method. Transfer molding is a process through which an encapsulatingmaterial, such as a thermosetting material, is caused to flow into acavity formed by components of a mold. The thermosetting material entersinto the cavity and flows over the electronic device[s] that is[are]located within the cavity and is then “cured” so that the resin hardensinto a non-flowable state. Traditionally it has been important tocontrol the flow of the material into the cavity for a number ofreasons, including: to provide void-free fill over the electricaldevice, to control the flow of the material so as to not contaminateunwanted areas with the encapsulating material, and to control the flowof the material so as to not cause any wire displacement or other damageto the assembly.

Also, it is well known in the art to use mechanical clamping mechanismsas sealing devices in conjunction with the molding cavity to attempt tocontain the thermosetting resin within the cavity prior to curing.However, because mechanical clamps must be applied in a symmetricalfashion, i.e. equal and opposite clamping forces, the configuration ofthe molds is limited to symmetrical designs.

Therefore there is a need for an improved encapsulation method fortransfer molding electronic devices which provides a leak proof sealingmechanism for asymmetric designs while using current equipment and knownmaterials and techniques.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method ofencapsulating an article having first and second surfaces includes thefollowing steps: positioning a first molding section in a sealingrelationship with the first surface of the article; positioning a secondmolding section adjacent the second surface of the article; filling thefirst molding section thereby forcing the second surface of the articleinto a sealing engagement with the second molding section; and fillingthe second molding section. Additionally, the step of filling the firstand second molding sections may include the step of filling the firstand second molding sections with an encapsulating material chosen from aclass consisting of epoxies (including thermo-set resins), silicones,sycar, polyimides, and polyurethanes. Also, depending on the type ofencapsulating material that is chosen, an optional step of curing theencapsulating material may be required.

Another aspect of the present invention is to provide a method whichincludes the following steps: mounting an electrical device to a firstsurface of a flexible substrate such that leads from the electricaldevice extend to a second surface of the flexible substrate; connectingthe leads to traces formed in the second surface of the flexiblesubstrate; positioning a portion of the flexible substrate carrying theelectrical device within a molding cavity defined by first and secondmolding sections, the substrate forming a seal with the first moldingsection; forming a seal between the substrate and the second moldingsection by filling the first molding section; and filling the secondmolding section.

Another aspect of the present invention is to provide a method oftransfer molding, including the steps of: positioning a first portion ofencapsulated material on a first surface by transfer molding; andpositioning a second portion of encapsulated material, of smallersurface area than the first portion, on a second surface by transfermolding.

Another aspect of the present invention is to provide a mold, whichincludes: a first molding section and a second molding section. Thefirst and second molding sections form a cavity for receiving an articleto be encapsulated. The surface area of a first side of an articleexposed to the cavity is greater than the surface area of a second sideof the article exposed to the cavity. At least one gate for injectingencapsulating material into the cavity is provided.

Another aspect of the present invention is to provide a non-symmetrical,transfer molded, encapsulated article.

The present invention provides several advantages over prior arttechniques. Yields are increased in comparison to parts which are globtopped. The method of the present invention may be carried out usingexisting processes and materials having well known and understoodproperties. Additionally, existing molding equipment may be used.Assembly throughput is increased. Those advantages and benefits, andothers, will be apparent to those of ordinary skill in the art from theDescription of a Preferred Embodiment, herein below.

BRIEF DESCRIPTION OF THE DRAWINGS

For the present invention to be readily understood and practiced, theinvention will now be described, for purposes of illustration and notlimitation, in conjunction with the following figures wherein:

FIG. 1 illustrates a substrate which may be used in conjunction with themethod of the present invention;

FIG. 2 illustrates a portion of the substrate of FIG. 1 carrying anelectrical device to be encapsulated;

FIG. 3 illustrates the opposite side of the substrate illustrated inFIG. 2;

FIG. 4 illustrates first and second mold sections which define a moldcavity;

FIG. 5 illustrates the portion of the substrate shown in FIGS. 2 and 3positioned in the mold cavity;

FIG. 6 illustrates the sequence of filling the mold cavity;

FIGS. 7 and 8 are perspective and plan views, respectively, of a firstside of the portion of the substrate illustrated in FIGS. 2 and 3, afterencapsulation;

FIGS. 9 and 10 are perspective and plan views, respectively, of a secondside of the portion of the substrate illustrated in FIGS. 2 and 3, afterencapsulation;

FIGS. 11 and 12 are views taken along the lines XI and XII,respectively, in FIG. 10;

FIGS. 13A and 13B illustrate an asymmetric encapsulated device; and

FIG. 14 illustrates another gate arrangement which may be used with themethod of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates a substrate 10 which may be used in conjunction withthe method of the present invention. The description of the method ofthe present invention in connection with the substrate 10 is forpurposes of illustration only, and not limitation. It is anticipatedthat the method and molds of the present invention may be used toencapsulate a large variety of articles, both electrical andnon-electrical. In FIG. 1, the substrate is a known flexible substratesuitable for receiving electrical devices 12 in openings 14. Theelectrical device may be any type of device, although a memory device isshown in FIG. 1.

The electrical device 12 may be connected to a first side 16 of thesubstrate 10 by any known means, e.g. lamination, adhesion, etc. Thedevice 12 may be of a type in which electrical connections extend fromthe center of the device. The device 12 is positioned such that theelectrical connections 18 of the device 12 extend through opening 14 toa second side 20 of substrate 10, seen in FIG. 3. In the remainingpre-singulation figures, only portion 22 of substrate 10 is illustratedfor purposes of convenience. The reader will recognize that thedescription of the method and molds hereinafter with respect to portion22 is actually carried out “x” times, e.g., 8, 10, 12, etc. dependingupon the size and capacity of the encapsulating equipment.

In FIG. 2 it is seen that the device 12 is positioned such that opening14 is not completely blocked leaving a small opening referred to as asecondary gate 24. If the device 12 completely blocks opening 14, thenan alternate means of providing encapsulating material must be providedas described below.

In FIG. 3, the second side 20 of the portion 22 of the substrate 10 isillustrated. Second side 20 has traces 26 formed therein. The traces 26may be formed using a solder masking step as is known in the art. Solderballs 28 may be embedded in portion 22 to provide atermination/connection point for each of the traces 26. After the device12 is connected to the portion 22 and the electrical connections 18 areextending through opening 14, each electrical connection 18 is connectedto one of the traces using any known connection technique and machinery.

A mold 30 which may be used in conjunction with the method of thepresent invention is illustrated in FIG. 4. In FIG. 4, a first moldsection 32 and a second mold section 34 cooperate to define a moldcavity 36.

FIG. 5 illustrates the portion 22 positioned within cavity 36. Theportion 22 is positioned such that device 12 is entirely within theportion of the cavity 36 formed by the first mold section 32, although,for other devices that may not be the case. Similarly, opening 14, andthe connection of the electrical connections 18 to traces 26 ispositioned entirely within the portion of the cavity 36 formed by thesecond mold section 34.

The asymmetry of the mold sections 32, 34 can be clearly seen in FIGS. 4and 5. In this illustrated embodiment, the surface area of the moldsection 32 is approximately three times greater than a surface area ofthe mold section 34. Generally, the surface area (A16) of the first side16 of portion 22 facing mold cavity 36 must be greater than the surfacearea (A20) of the second side 20 of portion 22 facing mold cavity 36.That is A16>A20.

Another by-product of the asymmetry is that first mold portion 32 may besealed against the first surface 16 of portion 22 by applying clampingpressure in the four areas marked 38. No such seal can be formed at thistime between second mold portion 34 and the second surface 20 of portion22 because no clamping pressure can be exerted in opposition to the twoareas marked 40. Not shown in FIGS. 4 and 5 is a runner and primarygate, which is the mechanism for injecting the encapsulating materialinto the portion of the mold cavity formed by the first mold section 32.

Turning now to FIG. 6, the sequential flow of encapsulating materialinto the mold cavity 34 is shown. Encapsulating material flows underpressure through runner 42 into the portion of the mold cavity formed byfirst mold section 32. The pressure in runner 42 is designated P1 whilethe pressure in the portion of the mold cavity formed by first moldsection 32 is designated P2. The encapsulating material passes throughsecondary gate 24 at a pressure of P3 and into the portion of the moldcavity formed by second mold section 34 at a pressure of P4. Thepressure is controlled such that P1>P2>P3>P4.

In one embodiment, the encapsulation material is a thermo-set epoxyresin mixture and may be loaded under a pressure in the range of500-2000 psi. Fill times when the encapsulation material is a thermo-setepoxy resin mixture are on the order of 3-10 seconds. The injectionpressure and fill times are dependent upon the specific encapsulatingmaterial that is used.

As the encapsulation material fills the portion of the mold cavityformed by the first mold section 32, the portion 22 of the substrate 10bends or flexes under the pressure exerted by the encapsulationmaterial. The bending brings the second surface 20 into a sealingengagement with the second molding section 34. Thus, a seal isdynamically formed as a portion of the mold cavity formed by the firstmold section 32 is filled. In low-pressure applications, it isanticipated that a force may be exerted to cause the bending to takeplace.

The encapsulation material may be chosen from a class consisting ofepoxies (including thermo-set resins), silicones, sycar, polyimides, andpolyurethanes. These encapsulation materials are suitable for use whenencapsulating electronic components because they have low moisturepermeability, high mobile ions barriers, good UV-VIS and alpha particleprotection, favorable mechanical, electrical and physical properties, aswell as a low dielectric constant to reduce the device propagation delayand high thermal conductivity to dissipate heat generated by thedevices. The proper choice of encapsulation material can enhancereliability of the device and improve its mechanical and physicalproperties. An optional curing step may hereinafter be required,depending upon the choice of encapsulation material, followed by removalof the mold sections 32, 34.

After the mold sections 32, 34 are removed, the encapsulated electronicdevice appears as shown in FIGS. 7, 8, 9, and 10. FIGS. 7 and 8 show thefirst side 16 of portion 22 after removal of mold section 32. A firstportion of encapsulated material 33 is formed as a result of theencapsulation material being loaded into mold section 32. FIGS. 9 and 10show the second side 20 of portion 22 after removal of mold section 34.A second portion of encapsulated material 35 is formed as a result ofthe encapsulation material being loaded into mold section 34.

FIGS. 11 and 12 illustrate views taken along the lines XI and XII,respectively, in FIG. 10. FIG. 12 provides a view of runner 42 andprimary gate 44, which is the mechanism for injecting the encapsulationmaterial into the portion of the mold cavity formed by the first moldsection 32.

FIGS. 13A and 13B show the final product of the present invention aftersingulation, i.e. after portions of the substrate 22 are separated andthe runner 42 is removed.

FIG. 14 shows another gate arrangement which may be used with the methodof the present invention. The electrical device to be encapsulated is amemory device identical to the one as shown in previous FIGS. 1 through13B, although the electrical device may be any type of device. Theelectrical device 12 is positioned such that opening 14 (see FIG. 1) iscompletely blocked. Thus encapsulating material injected into first moldsection 32 from runner 42 and primary gate 44 is contained within firstmold section 32. That is, there is no connecting gate between first moldsection 32 and second mold section 34 through which encapsulationmaterial may flow. Encapsulating material may be injected directly intomold section 34 through a gate 46 and a runner, not shown. Theencapsulating material injected into mold section 34 is contained withinmold section 34.

While the present invention has been described in conjunction withpreferred embodiments thereof, those of ordinary skill will recognizethat many modifications and variations thereof are possible. Forexample, a limitless number of asymmetric mold designs are possible. Itis anticipated that the method of the present invention may be carriedout using a variety of encapsulating materials and commerciallyavailable injection molding machines. Also, numerous other gateconfigurations are possible. The foregoing description and the followingclaims are intended to cover all such modifications and variations.

1. A system comprising: a flexible substrate; a device mounted upon saidsubstrate; and an asymmetrical mold, said mold comprising: a firstmolding section; a second molding section, said first and second moldingsections forming a cavity for receiving at least a portion of saidflexible substrate carrying said device; said cavity comprising a firstdefined by said first molding section and a second cavity defined bysaid section molding section; said cavity designed such that a firstsurface area of the device exposed to said first cavity is greater thana second surface area of the device exposed to said second cavity; afirst gate operatively connected to said first cavity for injectingencapsulating material into said first cavity; and a second gate at aseparate location from the location of said first gate and operativelyconnecting said second cavity to said first cavity.
 2. The system ofclaim 1 designed to withstand a psi of approximately 500-2000 psi and afilling time of approximately 3-10 seconds.
 3. The system of claim 1wherein said device is an electrical device.
 4. The system of claim 1wherein said device is a memory device.
 5. A system comprising: aflexible substrate; a device mounted upon said substrate; and anasymmetrical mold, said mold comprising: a first molding section; asecond molding section, said first and second molding sections forming acavity for receiving at least a portion of said flexible substratecarrying said device; said cavity comprising a first cavity defined bysaid first molding section and a second cavity defined by said sectionmolding section; said cavity designed such that a first surface area ofthe device exposed to said first cavity is greater than a second surfacearea of the device exposed to said second cavity; a first gateoperatively connected to said first cavity for injecting encapsulatingmaterial into said first cavity; and a second gate operatively connectedto said second cavity for injecting encapsulating material into saidsecond cavity; wherein said first and second gates are configured forsequential operation.
 6. The system of a claim 5 designed to withstand apsi of approximately 500-2000 psi and a filling time of approximately3-10 seconds.
 7. The system of a claim 5 wherein said device is anelectrical device.
 8. The system of claim 5 wherein said device is amemory device.